Lubricating oils have been generally used in various machines and apparatuses. For example, the lubricating oils have been used in gas compressors in which air or a refrigerant is compressed. The gas compressors are classified into a volume type and a turbo type according to their working principle for increasing a gas pressure. The volume type compressors are further classified into reciprocating compressors and rotary compressors.
The rotary compressors have been extensively used in view of saving of resources, measures for reduction of noises and vibrations, working efficiency, etc., as compared to the conventional reciprocating compressors. On the other hand, the rotary compressors are used under more severe lubricating conditions in which the lubricating oils must be brought into contact with high-temperature or high-pressure air or gases, as compared to the reciprocating compressors. Therefore, there is a demand for compressor oils for the rotary compressors which have a high thermal stability and a high oxidation stability.
There is a recent tendency that the rotary compressors are further downsized, and more frequently operated under poor ambient environmental conditions, e.g., in an atmosphere of an oxidative gas such as SOx and NOx or in a cutting oil mist atmosphere. In such a case, the lubricating oils suffer from occurrence of varnishing which tends to cause deposition of the varnishing on an inside of the compressors or clogging of a filter therein for a very short period of time, resulting in malfunction of the compressors.
On the other hand, a phenol-based antioxidant (di-t-butyl-p-cresol) which has been generally extensively used in the lubricating oils is likely to be volatilized and tends to suffer from considerable degree of discoloration. Therefore, the use of such a phenol-based antioxidant is not fully desirable from the viewpoints of a durability of the lubricating oils as well as suppression of discoloration of the oils. In addition, the phenol-based antioxidant or an amine-based antioxidant is insufficient in friction characteristics (such as anti-seizing property and wear resistance) by itself. Therefore, a friction controller containing sulfur/phosphorus having poor heat resistance and oxidation stability must be used in combination with the above antioxidant in the lubricating oils in order to improve a lubricating property thereof.
Under these circumstances, there is a demand for lubricating oils which are suitably used in machines or apparatuses operated under severe conditions such as gas compressors, excellent in heat resistance and oxidation stability, and hardly suffer from discoloration or occurrence of varnishing. For example, Patent Document 1 discloses a lubricating oil composition in which a phosphorus-containing phenol-based antioxidant, a phosphorus-free phenol-based antioxidant and an amine-based antioxidant are used in combination with each other. Patent Document 2 discloses a lubricating oil composition in which phenyl-α-naphthyl amine, p,p′-dialkyl diphenyl amine and a phosphorus-based extreme-pressure additive are used in combination with each other. Patent Document 3 discloses a lubricating oil composition in which a specific amine-based antioxidant and a phosphorus-containing phenol-based antioxidant are used in combination with each other.
On the other hand, in lubricating oils for internal combustion engines such as gasoline engines, diesel engines and gas engines, zinc dithiophosphate (Zn-DTP) has been used as both of an anti-wear agent and an antioxidant for a long time, and is also presently considered to be an important essential additive for the lubricating oils for internal combustion engines.
However, since the zinc dithiophosphate contains a large amount of a phosphorus component and a sulfur component in addition to a metal component (zinc) in a molecule thereof, sulfuric acid or phosphoric acid tends to be generated from the zinc dithiophosphate when decomposed. For this reason, the zinc dithiophosphate tends to exhaust a basic compound contained in engine oils and promote degradation of the lubricating oils, which tends to result in extremely shortened oil replacement intervals (this phenomenon means that a so-called base number retention property of the oils is insufficient). In addition, the zinc dithiophosphate tends to generate a varnishing under high-temperature conditions and therefore cause problems such as deterioration in cleaning property of an inside of the engine.
Under such circumstances, it has been demanded to develop an anti-wear additive which can be used in the lubricating oils for internal combustion engines in place of the zinc dithiophosphate.
In engines for current automobiles, an oxidation catalyst, a three way catalyst, an NOx occlusion type reducing catalyst, a diesel particulate filter (DPF), etc., have been used to purify exhaust gases emitted therefrom. It is known that these catalysts used for purification of exhaust gases tend to be adversely affected by metal components, phosphorus components and sulfur components contained in the engine oils. Therefore, it has been required that these components are reduced to prevent deterioration of the catalysts.
In consequence, there is an intense demand for lubricating oils for internal combustion engines which are excellent in properties basically required for lubricating oils for internal combustion engines (such as wear resistance, detergency and base number retention property) although they have a low metal content (i.e., a low sulfated ash content), a low phosphorus content and further a low sulfur content.
To solve the above problems, there have been conventionally proposed various additives for lubricating oils and various lubricating oil compositions. For example, Patent Document 1 has proposed the lubricating oil composition containing a specific phosphorus-containing phenol-based antioxidant. However, such a phosphorus-containing phenol-based antioxidant tends to be unsatisfactory in dissolvability in a base oil and therefore must be further improved to solve the above problems.
Patent Document 1:JP-A 11-35962
Patent Document 2: JP-A 2005-239897
Patent Document 3: JP-A 2007-161773